Two-dimensional spatial light modulators are widely used in a range of imaging applications from projection of color images to printing of monochrome and color images onto photosensitive media. Because it forms a complete, two-dimensional image at one time without requiring mechanical movement, the spatial light modulator offers a number of advantages over other types of imaging devices, such as scanning lasers, for example.
A spatial light modulator can be considered essentially as a two-dimensional array of light-valve elements, each element corresponding to an image pixel. Each array element is separately addressable and digitally controlled to modulate light by transmitting (or reflecting) or by blocking transmission (or reflection) of incident light from a light source. There are two salient types of spatial light modulators that are being employed for forming images in projection and printing apparatus. The liquid crystal device (LCD) modulates an incident beam by selectively altering the polarization of light for each pixel. An LCD may be transmissive, operating by selectively transmitting the incident beam through individual array elements. Other types of LCD are reflective, selectively changing the polarization of a reflected beam at individual array elements. The second basic type of spatial light modulator currently in use is the digital micromirror device (DMD), as disclosed in U.S. Pat. No. 5,061,049. The DMD modulates by reflection of light at each individual pixel site.
Spatial light modulators were initially developed for display and digital projection applications. Examples include display apparatus such as those disclosed in U.S. Pat. No. 5,325,137 and in U.S. Pat. No. 5,743,610, and miniaturized image display, such as mounted within a helmet or supported by eyewear, as is disclosed in U.S. Pat. No. 5,808,800.
More recently, spatial light modulators have been used in printing apparatus, such as the area printer disclosed in U.S. Pat. No. 5,652,661. It is instructive to consider some of the more important differences between projection and printing requirements for spatial light modulator devices. Effective image projection requires that the image forming device provide high levels of brightness. In display presentation, the human eye is relatively insensitive to many types of image artifacts and aberrations, since the displayed image is continually refreshed and is viewed from a distance. Motion and change also help to minimize the effects of many types of image artifacts. High resolution is not a concern for projection applications, with 72 pixels per inch normally satisfactory for many types of images.
Image printing, meanwhile, presents a number of different problems. For example, when viewing output from a high-resolution printing system, the human eye is not nearly as “forgiving” to artifacts, aberrations, and non-uniformity, since irregularities in optical response are more readily visible and objectionable on printed output. To obtain sufficiently high resolution, print output at 1200 dpi or higher may be necessary, depending on the application.
Conventional printing apparatus match system and optics to one type of photosensitive medium and to one image size format. For example, a particular printer may be designed to print standard 3.5×5 inch or 4×6 inch photographic prints. This is, in part, due to constraints imposed by conventional optical imaging methods that use film negatives. In order to provide acceptable throughput performance with conventional systems, it is necessary to fix the optical focus position, the position and aspect ratio of the negative, and the location of paper at the exposure plane. Thus, it is difficult to adapt conventional printing apparatus optics for providing, on a photosensitive medium, prints having more than one format. At the same time, however, spatial light modulators are not under similar constraints; it is possible to form two-dimensional images having, in units of pixels, a selectable number of rows and columns, thereby allowing a single spatial light modulator to form images in different aspect ratios.
Zoom lenses are one type of conventional optical solution that allows a single printing apparatus to provide output images in multiple output formats. However, such a solution is costly and requires either time-consuming manual focus adjustment for switching between output size formats or a complex motor assembly for automatic focus adjustment.
Thus, it can be seen that there would be advantages to a spatial light modulator-based imaging system that prints, onto a photosensitive medium, images in a selectable number of output size formats, with an optical system that easily adjusts between output size formats.